diff --git a/src/qibo/hamiltonians/hamiltonians.py b/src/qibo/hamiltonians/hamiltonians.py
index 2c527df0ba..8e17e88fbb 100644
--- a/src/qibo/hamiltonians/hamiltonians.py
+++ b/src/qibo/hamiltonians/hamiltonians.py
@@ -250,7 +250,8 @@ def __mul__(self, o):
             if self.backend.np.real(o) >= 0:  # TODO: check for side effects K.qnp
                 r._eigenvalues = o * self._eigenvalues
             elif not self.backend.issparse(self.matrix):
-                r._eigenvalues = o * self.backend.np.flip(self._eigenvalues, (0,))
+                axis = (0,) if isinstance(self.backend, PyTorchBackend) else 0
+                r._eigenvalues = o * self.backend.np.flip(self._eigenvalues, axis)
         if self._eigenvectors is not None:
             if self.backend.np.real(o) > 0:  # TODO: see above
                 r._eigenvectors = self._eigenvectors